The present invention relates to apparatus and methods for axially and non-rotationally fixing one or more spacers and a water rod to one another in a nuclear fuel bundle assembly and particularly relates to spacer/water rod capture apparatus and methods for assembly and disassembly of the water rods and spacers.
In boiling water nuclear reactors, fuel rods are grouped together in an essentially open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. The fuel rods are supported in each assembly between upper and lower tie plates. One or more water rods are usually located interiorly within each fuel rod bundle. Spacers are employed at predetermined elevations along each fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation.
Typical spacers often include a plurality of ferrules arranged in side-by-side relation and secured, for example, by welding to one another to form a support matrix for the fuel rods. Generally, the role of the spacer in a fuel bundle assembly is to maintain the rods in fixed lateral locations relative to one another. These spacers are secured or captured in a manner preventing their movement relative to the longitudinal axis of the bundle during operation of the reactor. Conventional methods of maintaining the fuel rod spacers in position along the axis of the bundle include the use of a pair of tabs welded to the water rods engaging a spacer along its top and bottom, preventing axial movement of the spacer relative to the water rod in either axial direction. These spacers are assembled onto the water rod by relatively rotating the spacers and water rod such that the tabs pass through an opening in the spacer structure. The spacer is then rotated to an orientation such that part of the spacer structure is axially contacting one of the two tabs, the lower tab being preferred. In the assembled bundle, the water rod is typically prevented from rotation by a square lower end plug received in a square hole in the lower tie plate.
There are, however, occasions where the fuel bundle is unchanneled. By unchanneled is meant that the fuel bundle assembly does not have the surrounding channel. For example, the bundles are unchanneled during initial fabrication when the spacers are secured to the water rod. Unchanneled bundles are relatively flexible with respect to torsion about the bundle axis. Thus, twist can occur in unchanneled bundles from normal handling of the bundles. If the twist is large enough, certain of the spacers, e.g., the upper spacers can be rotated to such an extent that the water rod tabs locate over and under the openings in the spacer structure used during assembly and no longer provide axial restraint. Also, for irradiated bundles where the friction forces between the spacer springs and the rod is significantly reduced, the spacers can become axially displaced from their proper location. Further, current designs employ square water rod lower end plugs and square lower tie plate holes to maintain angular orientation of the water rod tab relative to the spacer structure. Such end plugs and holes are costly as compared with round end plugs and holes.
In accordance with the present invention, a spacer and water rod retention or capture assembly is provided in which a single tab on the water rod is captured by spacer structure, preventing relative axial displacement of the spacer and water rod, notwithstanding twisting of the fuel bundle, particularly in the unchanneled configuration. The retention assembly prevents the rotation of the water rod and the tab carried thereby relative to the spacer to the extent the water rod tab cannot be aligned with an opening in the spacer which would otherwise permit relative axial displacement of the spacer and water rod.
In a first preferred embodiment of the present invention, the spacer includes a spacer structure or clip having a central opening bounded perimetrically on axially opposite ends and laterally by opposite side portions. One side portion of the clip includes a tab projecting generally toward the opposite side portion and which tab is resiliently mounted, delimiting the slot opening. The clip, of course, is welded or otherwise secured to the spacer and the slot lies adjacent the water rod position in a plane angularly related to the water rod. Upon locating the water rod in the spacer, the water rod and spacer are relatively rotated such that the water rod tab engages and resiliently displaces the clip tab enabling the water rod tab to pass the clip tab into the slot. The clip tab then resiliently reverts to its original shape. The interference with the resilient tab, together with the obtuse angle of approach of the water rod tab to the clip tab, prevents disengagement of the water rod tab from the spacer. To disassemble the spacer and water rod, a tool can be inserted to deflect the clip tab in the same direction as it is deflected in the initial assembly by the water rod tab whereby the water rod tab can be rotated out of the slot freeing the water rod and clip for relative axial movement.
In a second preferred embodiment of the present invention, the water rod tab is rotated into a slot in the spacer structure whereby one side edge of the slot forms a limit stop for the tab. The water rod tab is prevented from rotating out of the slot by the installation of a second water rod adjacent the first water rod. In the first embodiment, the water rod tab is restrained from rotation by its fit-up with the slot of the spacer clip and in the second embodiment by the spacer clip and an adjoining water rod. In both embodiments, reliance on a square lower end plug and correspondingly shaped tie plate hole to prevent water rod rotation as in certain current spacer/water rod retention assemblies is entirely avoided.
In a preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a plurality of generally parallel, axially extending fuel rods, a plurality of spacers axially spaced from one another for maintaining the fuel rods spaced laterally from one another, and a pair of water rods extending in generally side-by-side parallel relation to the fuel rods and passing through the spacers, a spacer and water rod retention assembly, comprising a water rod connecting structure carried by one of the spacers and having a slot bounded by opposite structure portions spaced in an axial direction from one another and a pair of oppositely disposed side structure portions between the axially opposite portions perimetrically enclosing the slot, the slot substantially spanning between the water rods, one of the water rods having a tab projecting laterally outwardly thereof and into the slot, the tab having an axial extent engaging the axially opposite structure portions to limit relative axial movement of the one spacer and the one water rod, another of the water rods and one of the side structure portions confining the water rod tab in the slot precluding substantial relative rotational movement between the one spacer and the one water rod.
In a further preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a plurality of generally parallel, axially extending fuel rods, a plurality of spacers axially spaced from one another for maintaining the fuel rods spaced laterally from one another, and a pair of water rods extending in generally side-by-side parallel relation to the fuel rods and passing through the spacers, a spacer and water rod retention assembly, comprising a water rod connecting structure carried by one of the spacers and having a slot bounded by opposite end portions spaced in an axial direction from one another and at least one side portion between the axially opposite end portions, the slot substantially spanning between the water rods, one of the water rods having a tab projecting laterally outwardly thereof and into the slot, the tab having an axial extent engageable with the axially opposite end portions to limit relative axial movement of the one spacer and the one water rod, another of the water rods and the one side portion confining the water rod tab in the slot precluding substantial relative rotational movement between the one spacer and the one water rod.
In a further preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a plurality of spacers for maintaining fuel rods spaced one from the other and a pair of water rods extending axially and generally perpendicular to and through the spacers, a method of retaining the spacer on one of the water rods, comprising the steps of relatively rotating one of the water rods and one of the spacers to dispose a tab projecting laterally outwardly of the one water rod into a slot carried by a clip on the one spacer, providing a stop along one side of the slot of the clip to preclude continued rotation of the tab and clip relative to one another and installing another of the water rods adjacent the one water rod and adjacent the clip to preclude relative rotation of the one water rod and the spacer in a direction removing the tab from the slot whereby relative movement of the one spacer and the one water rod in opposite axial directions and substantial relative rotational movement between the one water rod and the one spacer are precluded.